In an internal combustion engine (hereinafter also referred to as an engine) mounted on an automobile or similar vehicle, an amount (a fuel injection amount) of fuel to be injected from an injector (a fuel injection valve) is feedback-controlled (air-fuel ratio feedback control) based on a deviation between an air-fuel ratio (an actual air-fuel ratio) detected by an air-fuel ratio sensor disposed at an exhaust passage and a target air-fuel ratio (such as an ideal air-fuel ratio). Performing this air-fuel ratio feedback control allows accurate control of the air-fuel ratio, and improves exhaust emission.
Incidentally, in a multicylinder internal combustion engine that includes a plurality of cylinders, an air-fuel ratio control is usually performed on all cylinders using the same control variable. Accordingly, even if the air-fuel ratio control is performed, the actual air-fuel ratio may be varied among the cylinders (A/F imbalance). In the case where the degree of the variation in air-fuel ratio is small, the air-fuel ratio feedback control can absorb the variation. Additionally, a catalyst can also purify a harmful component in exhaust gas. Therefore, this does not affect the exhaust emission, and is not particularly problematic. However, for example, in the case where a fuel injection system in a part of the plurality of cylinders of the internal combustion engine breaks down and the air-fuel ratios among the cylinders vary considerably, the exhaust emission becomes deteriorated (for example, see Patent Literature 1). From this aspect, in the field of automobiles, in order to prevent running in a state of deteriorated exhaust gas, detection of abnormal air-fuel ratio variation (A/F imbalance) among the cylinders in a vehicle (OBD: On Board Diagnosis) has been defined by law.
In contrast, in the internal combustion engine mounted on the automobile and similar vehicle, a supercharger (hereinafter also referred to as a turbocharger) using energy of exhaust air is equipped. The turbocharger generally includes a turbine wheel, a compressor impeller, and a coupling shaft. The turbine wheel rotates by exhaust gas flowing through the exhaust passage of the internal combustion engine. The compressor impeller forcibly sends air inside of an intake passage into a combustion chamber of the engine. The coupling shaft couples the turbine wheel and the compressor impeller together. In the turbocharger with this structure, the turbine wheel disposed at the exhaust passage rotates by energy from the exhaust air. Along with this rotation, the compressor impeller disposed at the intake passage rotates to supercharge intake air. Subsequently, the supercharged air is forcibly sent into the combustion chamber of each cylinder in the engine.
As a turbocharger, a twin-entry turbocharger (for example, see Patent Literature 2 and 3) that includes two exhaust gas inflow ports at the turbine (the turbine housing) is known. This twin-entry turbocharger is employed in order to prevent mutual interference of exhaust gas pulsations in the multicylinder internal combustion engine, to ensure high output power, and similar purpose. A multicylinder internal combustion engine (a supercharger-equipped multicylinder internal combustion engine) with the twin-entry turbocharger is configured as follows. A plurality of cylinders are divided into cylinder groups where respective exhaust strokes are not adjacent to one another. Exhaust gas for each cylinder group is independently guided to the exhaust gas inflow port of the turbine.